The present invention is related to devices serving to detect minute vibrations and sound waves in air, best known as microphones. The invention especially utilizes the unique properties of coherent, monochromatic light, best known as laser light, to create interference effects between two aligned beams of light.
Such effects are well known from the Michelson Interferometer which has been described extensively in the literature of physics, such as Scientific American: Lasers and Light by A. L. Schawlow, 1969, or from the Fizeau Air Wedge experiment, described for example in Physical Education by P. W. Fish, January, 1971.
Briefly stated, two aligned beams of laser light, of which one beam is slightly delayed in relation to the other beam of the same frequency, will cause the two beams to reinforce each other if they are in the same phase or to cancel each other if one beam is 180.degree. out of phase.
If one of the two beams is reflected by an object in motion such that the direction of motion is generally in the same direction as the non-reflected stationary beam, and the two beams are aligned by means of suitable mirrors into a single beam, the resulting interference pattern will move at a velocity that is twice the velocity of the moving object along the axis of the aligned beam. As the interference pattern moves in the direction of the aligned beam, a light sensor placed in the path of the beam will sense light intensity variations that vary as a function of the movements of the reflecting object.
The interference caused by beams of light has been used by inventors to construct microphones that are very sensitive and have other qualities. As examples, U.S. Pat. No. 3,470,329 by N. O. Young issued Sept. 30, 1969 entitled Interferometer Microphone describes a microphone based on interferences created by light reflected from a low-mass membrane. U.S. Pat. No. 1,709,762 by V. K. Zworykin, issued Apr. 16, 1929, entitled Interferometer Microphone describes the interference of light that is traversing an air-space also traversed by acoustic waves which modulate the light beam.
The present invention utilizes the availability of coherent, monochromatic light sources that are now available in the form of small relatively inexpensive lasers to produce a microphone that is more compact and less complex in construction and which provides additional advantages as described in the course of the following specification with appended drawings.
It is a primary object of the invention to produce a microphone that provides a high degree of fidelity in the transformation of sound waves to electrical signals.
It is another important object of the invention to produce a microphone that is compact in size.
It is a further object of the invention to produce a microphone that has multi-channel capability such as to provide sound reproduction that is closely correlated to the original sound patterns in a locale with sound reflecting properties.
It is still another object of the invention to provide a microphone that is dependable in operation and that is capable of mass production and without undue complexities.
Other objects and advantages of the invention will become apparent in the course of the following description with its appended drawings.